An anaerobic organism or anaerobe is any organism that does not
require oxygen for growth. It may react negatively or even die if
oxygen is present. (In contrast, an aerobic organism (aerobe) is an
organism that can survive and grow in an oxygenated environment.)
An anaerobic organism may be unicellular (e.g. protozoans,[1]
bacteria[2]) or multicellular.[3] For practical purposes, there are
three categories of anaerobe: obligate anaerobes, which are harmed by
the presence of oxygen; aerotolerant organisms, which cannot use
oxygen for growth but tolerate its presence; and facultative
anaerobes, which can grow without oxygen but use oxygen if it is
present.

Contents

1 First observation
2 Classification
3 Energy metabolism

3.1 Fermentation

4 Culturing anaerobes
5 Multicellularity
6 References

First observation[edit]
In his letter of 14 June 1680 to The Royal Society, Antonie van
Leeuwenhoek described an experiment he carried out by filling two
identical glass tubes about halfway with crushed pepper powder, to
which some clean rain water was added. Van Leeuwenhoek sealed one of
the glass tubes by using a flame and left the other glass tube open.
Several days later, he discovered in the open glass tube ‘a great
many very little animalcules, of divers sort having its own particular
motion.’ Not expecting to see any life in the sealed glass tube, Van
Leeuwenhoek saw to his surprise ‘a kind of living animalcules that
were round and bigger than the biggest sort that I have said were in
the other water.’ The conditions in the sealed tube had become quite
anaerobic owing to consumption of oxygen by aerobic microorganisms.[4]
In 1913
Martinus BeijerinckMartinus Beijerinck repeated Van Leeuwenhoek's experiment and
identified
Clostridium butyricumClostridium butyricum as a prominent anaerobic bacterium in
the sealed pepper infusion tube liquid. Beijerinck commented:

'We thus come to the remarkable conclusion that, beyond doubt, Van
Leeuwenhoek in his experiment with the fully closed tube had
cultivated and seen genuine anaerobic bacteria, which would happen
again only after 200 years, namely about 1862 by Pasteur. That
Leeuwenhoek, one hundred years before the discovery of oxygen and the
composition of air, was not aware of the meaning of his observations
is understandable. But the fact that in the closed tube he observed an
increased gas pressure caused by fermentative bacteria and in addition
saw the bacteria, prove in any case that he not only was a good
observer, but also was able to design an experiment from which a
conclusion could be drawn.' [4]

Classification[edit]

Aerobic and anaerobic bacteria can be identified by growing them in
test tubes of thioglycollate broth:
1: Obligate aerobes need oxygen because they cannot ferment or respire
anaerobically. They gather at the top of the tube where the oxygen
concentration is highest.
2: Obligate anaerobes are poisoned by oxygen, so they gather at the
bottom of the tube where the oxygen concentration is lowest.
3:
Facultative anaerobesFacultative anaerobes can grow with or without oxygen because they
can metabolise energy aerobically or anaerobically. They gather mostly
at the top because aerobic respiration generates more adenosine
triphosphate (ATP) than either fermentation or anaerobic respiration.
4: Microaerophiles need oxygen because they cannot ferment or respire
anaerobically. However, they are poisoned by high concentrations of
oxygen. They gather in the upper part of the test tube but not the
very top.
5:
AerotolerantAerotolerant organisms do not require oxygen as they metabolise
energy anaerobically. Unlike obligate anaerobes however, they are not
poisoned by oxygen. They can be found evenly spread throughout the
test tube.

For practical purposes, there are three categories of anaerobe:

Obligate anaerobes, which are harmed by the presence of oxygen.[5][6]
Two examples of obligate anaerobes are Clostridium botulinum and the
bacteria which live near hydrothermal vents on the deep-sea ocean
floor.
AerotolerantAerotolerant organisms, which cannot use oxygen for growth, but
tolerate its presence.[7]
Facultative anaerobes, which can grow without oxygen but use oxygen if
it is present.[7]

Energy metabolism[edit]
Some obligate anaerobes use fermentation, while others use anaerobic
respiration.[8]
AerotolerantAerotolerant organisms are strictly fermentative.[9]
In the presence of oxygen, facultative anaerobes use aerobic
respiration; without oxygen, some of them ferment; some use anaerobic
respiration.[7]
Fermentation[edit]
There are many anaerobic fermentative reactions.
Fermentative anaerobic organisms mostly use the lactic acid
fermentation pathway:

C6H12O6 + 2 ADP + 2 phosphate → 2 lactic acid + 2 ATP

The energy released in this equation is approximately 150 kJ per mol,
which is conserved in regenerating two ATP from ADP per glucose. This
is only 5% of the energy per sugar molecule that the typical aerobic
reaction generates.
Plants and fungi (e.g., yeasts) in general use alcohol (ethanol)
fermentation when oxygen becomes limiting:

C6H12O6 (glucose) + 2 ADP + 2 phosphate → 2 C2H5OH + 2 CO2↑ + 2
ATP

The energy released is about 180 kJ per mol, which is conserved in
regenerating two ATP from ADP per glucose.
Anaerobic bacteria and archaea use these and many other fermentative
pathways, e.g., propionic acid fermentation, butyric acid
fermentation, solvent fermentation, mixed acid fermentation,
butanediol fermentation, Stickland fermentation, acetogenesis, or
methanogenesis.
Culturing anaerobes[edit]
Since normal microbial culturing occurs in atmospheric air, which is
an aerobic environment, the culturing of anaerobes poses a problem.
Therefore, a number of techniques are employed by microbiologists when
culturing anaerobic organisms, for example, handling the bacteria in a
glovebox filled with nitrogen or the use of other specially sealed
containers, or techniques such as injection of the bacteria into a
dicot plant, which is an environment with limited oxygen. The GasPak
System is an isolated container that achieves an anaerobic environment
by the reaction of water with sodium borohydride and sodium
bicarbonate tablets to produce hydrogen gas and carbon dioxide.
HydrogenHydrogen then reacts with oxygen gas on a palladium catalyst to
produce more water, thereby removing oxygen gas. The issue with the
Gaspak method is that an adverse reaction can take place where the
bacteria may die, which is why a thioglycollate medium should be used.
The thioglycollate supplies a medium mimicking that of a dicot, thus
providing not only an anaerobic environment but all the nutrients
needed for the bacteria to thrive.[10]
Multicellularity[edit]
Except for three species of anaerobic loricifera, all known complex
multicellular life is aerobic, i.e. needs oxygen to survive.
In 2010 three species of anaerobic loricifera were discovered in the
hypersaline anoxic
L'Atalante basin at the bottom of the Mediterranean
Sea. They lack mitochondria which contain the oxidative
phosphorylation pathway, which in all other animals combines oxygen
with glucose to produce metabolic energy, and thus they consume no
oxygen. Instead these loricifera derive their energy from hydrogen
using hydrogenosomes.[11][3]
Some organisms metabolise primarily using glycogen, for example the
Nereid (worm)s and some polychaetes,[12] or the juvenile form of the
pork parasite
Trichinella spiralisTrichinella spiralis which causes trichinosis.[13]
References[edit]